Determination system and determination method for determining whether metal lithium is preciptated in a lithium ion secondary battery, and vehicle equipped with the determination system

ABSTRACT

A determination system for determining whether metal lithium is precipitated in a lithium ion secondary battery includes: a discharging unit that causes the lithium ion secondary battery to perform constant current discharge until a voltage of the lithium ion secondary battery becomes a voltage corresponding to a predetermined low state of charge; a natural increase acquisition unit that acquires a natural increase in voltage of the lithium ion secondary battery after the constant current discharge is terminated; and a precipitation determining unit the compares the acquired natural increase with a predetermined threshold, that determines that the metal lithium is not precipitated when the natural increase is larger than or equal to the threshold, and that determines that the metal lithium is precipitated when the natural increase is smaller than the threshold.

INCORPORATION BY REFERENCE

The disclosure of Japanese Patent Application No. 2010-035727 filed onFeb. 22, 2010 including the specification, drawings and abstract isincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a determination system and determination methodfor determining whether metal lithium is precipitated in a lithium ionsecondary battery, and a vehicle equipped with the determination system.

2. Description of the Related Art

Generally, the internal state of a lithium ion secondary battery isinspected. As one internal state of the lithium ion secondary battery,it is intended to recognize through inspection whether metal lithium isprecipitated. For example, a phenomenon called “dendrite precipitation”in Japanese Patent Application Publication No. 8-190934 (JP-A-8-190934)is also the precipitation of metal lithium. A lithium ion secondarybattery originally does not contain metal lithium; however, depending onusage, metal lithium may precipitate on a surface of a negativeelectrode. A lithium ion secondary battery that has reached such a stateneeds to be replaced because deterioration in performance is remarkable.Therefore, it is necessary to recognize whether metal lithium isprecipitated in a lithium ion secondary battery.

A method of detecting the internal state of a lithium ion secondarybattery is, for example, described in Japanese Patent ApplicationPublication No. 2003-59544 (JP-A-2003-59544). In the method described inJP-A-2003-59544, the lithium ion secondary battery first undergoesconstant current charge and subsequently undergoes constant voltagecharge. Then, a difference in index, such as charged capacity andinternal resistance, between the charged battery and a standard batteryis obtained. The internal state of the inspected battery is intended tobe recognized on the basis of the above difference.

However, in order to apply the above related art, a power supply forcharging an inspected battery needs to be compatible with both constantcurrent charge and constant voltage charge. For this reason, a complexpower supply system is required. Particularly, for example, in the caseof in-car application, it is difficult to perform required charge by anelectrical system of a vehicle itself. Generally, the electrical systemof a vehicle is not designed for constant voltage control.

In addition, information about a decrease in capacity or an increase ininternal resistance may be acquired as a result of inspection; however,it is impossible to determine what factor gives the acquiredinformation. That is, not only in the case of precipitation of metallithium but also in the case of normal usage degradation (hereinafter,referred to as “cycle degradation”) that is not attended withprecipitation of metal lithium, the tendency of a decrease in capacityor an increase in internal resistance is observed. Therefore, it hasbeen difficult to determine whether metal lithium is precipitated.

SUMMARY OF INVENTION

The invention provides a determination system and determination methodfor determining whether metal lithium is precipitated in a lithium ionsecondary battery without using constant voltage control, and a vehicleequipped with the determination system.

A first aspect of the invention provides a determination system fordetermining whether metal lithium is precipitated in a lithium ionsecondary battery. The determination system includes: a discharging unitthat causes the lithium ion secondary battery to perform constantcurrent discharge until a voltage of the lithium ion secondary batterybecomes a voltage corresponding to a predetermined low state of charge;a natural increase acquisition unit that acquires a natural increase involtage of the lithium ion secondary battery after the constant currentdischarge is terminated; and a precipitation determining unit thecompares the acquired natural increase with a predetermined threshold,that determines that the metal lithium is not precipitated when thenatural increase is larger than or equal to the threshold, and thatdetermines that the metal lithium is precipitated when the naturalincrease is smaller than the threshold.

A second aspect of the invention provides a vehicle that includes: alithium ion secondary battery; and the determination system according tothe first aspect.

A third aspect of the invention provides a determination method fordetermining whether metal lithium is precipitated in a lithium ionsecondary battery. The determination method includes: causing thelithium ion secondary battery to perform constant current discharge;terminating the constant current discharge when the lithium ionsecondary battery has a predetermined low state of charge through theconstant current discharge; acquiring a natural increase in voltage ofthe lithium ion secondary battery after the constant current dischargeis terminated; comparing the acquired natural increase with apredetermined threshold; and determining that the metal lithium is notprecipitated when the natural increase is larger than or equal to thethreshold, and determining that the metal lithium is precipitated whenthe natural increase is smaller than the threshold.

In a battery in which metal lithium is precipitated, polarization of anegative electrode is remarkable when the SOC is low, so polarization ofthe negative electrode becomes large while polarization of a positiveelectrode is not so large. Therefore, only the polarization of thenegative electrode contributes to an increase in internal resistancewhen the SOC is low. Therefore, in comparison with a battery in whichmetal lithium is not precipitated, the internal resistance is small whenthe SOC is low. In the aspects of the invention, this situation isdetermined on the basis of a natural increase in battery voltage afterthe battery is subjected to constant current discharge.

According to the aspects of the invention, it is possible to provide adetermination system and determination method for determining whethermetal lithium is precipitated in a lithium ion secondary battery withoutusing constant voltage control, and a vehicle equipped with thedetermination system.

BRIEF DESCRIPTION OF DRAWINGS

The features, advantages, and technical and industrial significance ofthis invention will be described below with reference to theaccompanying drawings, in which like numerals denote like elements, andwherein:

FIG. 1 is a block diagram of a lithium precipitation determinationsystem according to an embodiment of the invention;

FIG. 2 is a flowchart of a determination procedure executed by thelithium precipitation determination system according to the embodimentof the invention;

FIG. 3 is a graph that shows changes in voltage of both ends of abattery in the determination procedure according to the embodiment ofthe invention;

FIG. 4 is a graph that shows the relationship between an SOC (state ofcharge) and potentials of positive and negative electrodes in a lithiumion secondary battery according to the embodiment of the invention; and

FIG. 5 is a perspective view of a hybrid automobile equipped with thelithium precipitation determination system shown in FIG. 1.

DETAILED DESCRIPTION OF EMBODIMENTS

Hereinafter, an embodiment of the invention will be described in detailwith reference to the accompanying drawings. A lithium precipitationdetermination system 1 for a lithium ion secondary battery according tothe present embodiment is configured as shown in FIG. 1. The lithiumprecipitation determination system 1 for a lithium ion secondary batteryshown in FIG. 1 determines whether metal lithium is precipitated in abattery group 5. The battery group 5 is a battery pack in which aplurality of lithium ion secondary batteries are serially connected.

The lithium precipitation determination system 1 includes an ammeter 2,a voltmeter 3, a processing unit 4, a load 7 and a thermometer 11. Theammeter 2 measures the magnitude of current that flows through thebattery group 5. The voltmeter 3 measures the voltage between both endsof the battery group 5. The processing unit 4 acquires the resultsmeasured by the voltmeter 3, and the like, and then makes the abovedescribed determination or performs processing necessary for thedetermination on the basis of the measured results. The processing unit4 also controls the load 7. The thermometer 11 acquires the temperatureof the battery group 5.

The load 7 includes a charging unit 8, a discharging unit 9 and a DC/DCconverter 10. The charging unit 8 functions to supply charging currentto the battery group 5. The charging unit 8 is, for example, agenerator. The discharging unit 9 receives discharging current from thebattery group 5 to operate in some way. The discharging unit 9 is, forexample, a motor. One device may serve as both the charging unit 8 andthe discharging unit 9. The DC/DC converter 10 uses the discharging unit9 to discharge current as constant current discharge, which will bedescribed later. In addition, a relay 6 is arranged between the load 7and the battery group 5. The relay 6 is also operated by the processingunit 4.

Determination is made by the lithium precipitation determination system1 shown in FIG. 1 in accordance with the procedure shown in theflowchart of FIG. 2. First, constant current discharge is started (S1).That is, the relay 6 is connected, and the discharging unit 9 isoperated by discharging current from the battery group 5. At this time,the status of operation of the discharging unit 9 is controlled whilemonitoring the ammeter 2 so that the discharging current is constant.

The current value of the constant current discharge is set to fallwithin the range that does not degrade the batteries of the batterygroup 5. In order not to degrade the batteries of the battery group 5 inconstant current discharge, the current value should be set to a valuein ampere that is lower than or equal to three times of the value thatindicates the battery capacity of the battery group 5 in Ah (amperehour). For example, when the battery capacity of the battery group 5 is5 Ah, the current value of constant current discharge should be lowerthan or equal to 15 A. The current having the above current value almostdoes not adversely influence the service life of the batteries. Inaddition, the current value should be set to a value that is higher thanor equal to one-tenth of the value that indicates the battery capacityof the battery group 5 in Ah (ampere hour). This is because an extremelylow current causes an increased period of time for determination.

Note that, when the voltage between both ends of the battery group 5before constant current discharge is started in step S1 is too low, thebattery group 5 may be preliminary charged to some degree ahead ofconstant current discharge. In this case, a threshold for determiningwhether to perform preliminary charge (preliminary charge determinationthreshold) is set for the preliminary voltage of both ends of thebattery group. For the threshold, for example, it is conceivable that avoltage corresponding to the 60 to 70 percent SOC (state of charge) isused as a determination start allowable voltage. Then, when thepreliminary voltage of both ends of the battery group 5 is higher thanor equal to the determination start allowable voltage, the constantcurrent discharge is immediately started; whereas, when the preliminaryvoltage of both ends of the battery group 5 is lower than thedetermination start allowable voltage, charging is performed. When thepreliminary charge determination threshold is too low, constant currentdischarge cannot be sufficiently performed, and there is a possibilitythat the determination accuracy for the lithium precipitationdetermination system 1 cannot be sufficiently obtained. On the otherhand, when the preliminary charge determination threshold is too high,an unnecessary process is performed, and it takes an extra period oftime for determination.

In addition, a target charge voltage for preliminary charge is desirablyset to a voltage that ranges from a value that is equal to thepreliminary charge determination threshold (a voltage corresponding tothe 60 to 70 percent SOC (state of charge)) to a value that is higher bythe 10 percent SOC than the preliminary charge determination threshold.This is because, as the target charge voltage is too low, the voltage ofthe batteries at the time of start of constant current discharge isinsufficient and, as a result, measurement accuracy deteriorates. Inaddition, this is also because, as the target charge voltage is toohigh, it takes time for charging and constant current dischargethereafter. Charging is, of course, performed using the charging unit 8of the load 7.

While the constant current discharge started in step S1 is beingperformed, the voltage between both ends of the battery group 5 isconstantly monitored. This monitors whether the voltage between bothends has decreased to a predetermined discharge termination voltage VQ(S2). The discharge termination voltage VQ at this time is desirably avoltage corresponding to the 10 to 20 percent low SOC. If the dischargetermination voltage VQ is too high, appropriate determination cannot bemade as will be described later. On the other hand, if the dischargetermination voltage VQ is too low, the battery group 5 is brought intoan overdischarge state. That is, the determination process itselfdegrades the battery group 5.

When the voltage between both ends has decreased to the dischargetermination voltage VQ (S2: YES), the constant current discharge isterminated, and the relay 6 is opened (S3). Then, after that, thevoltage between both ends of the battery group 5 naturally increases tosome extent. Then, the amount of increase in the voltage, that is, thevoltage recovery amount VR, is acquired (S4). The details will bedescribed later. Then, the acquired voltage recovery amount VR iscompared with a threshold (lithium precipitation determinationthreshold) predetermined therefor (S5). When the acquired voltagerecovery amount VR is lower than or equal to the threshold (S5: YES), itis determined that metal lithium is precipitated in the battery group 5(S6). When the acquired voltage recovery amount VR is higher than thethreshold (S5: NO), it is determined that metal lithium is notprecipitated in the battery group 5 (S7). Up to this point, the basicprocedure of determination according to the present embodiment isdescribed.

Next, the process shown in the flowchart of FIG. 2 will be described infurther details with reference to FIG. 3. FIG. 3 is a graph that showschanges in voltage of the battery group 5 in process of executing theprocess shown in FIG. 2. The ordinate axis represents a cell voltage(voltage between both ends of each battery) [V], and the abscissa axisrepresents time [s]. The graph of FIG. 3 shows a curve X and a curve Y.The curve X is an example of a voltage change obtained when metallithium is not precipitated in the battery group 5. The curve Y is anexample of a voltage change obtained when metal lithium is precipitatedin the battery group 5. In FIG. 3, time Z at which the voltage is thelowest corresponds to the time at which the constant current dischargeis terminated in step S3 in FIG. 2. In FIG. 3, the curve X and the curveY are overlappingly shown so that the time at which the constant currentdischarge is terminated coincides with each other with respect to theabscissa axis. Then, the zero point of the abscissa axis is set at thetime at which the constant current discharge is started (correspondingto S1 in FIG. 2) in the curve X for the sake of convenience.

In FIG. 3, a portion before (on the left side of) time Z is a period oftime during which the constant current discharge is performed from stepS1 to step S3 in FIG. 2. In FIG. 3, the cell voltage at the time whenthe constant current discharge is started is set at 3.6 V, and thedischarge termination voltage VQ is set at 2.3 V in cell voltage. Duringthe constant current discharge, the cell voltage gradually decreases.The curve X and the curve Y slightly differ from each other in the shapeof the graph during the constant current discharge; however, thisdifference falls within the range of individual difference, and is not asignificant difference.

When the constant current discharge is terminated at time Z, the cellvoltage steeply increases thereafter. However, the cell voltage does notincrease without limit, the curve X converges to about 3.3 to 3.4 V, andthe curve Y converges to about 2.7 to 2.8 V. A difference between theconverged cell voltage and 2.3 V that is the cell voltage VQ at the timeof termination of discharge is the voltage recovery amount VR acquiredin step S4 in FIG. 2. That is, the voltage recovery amount VR of thecurve X is about 1.0 to 1.1 V, and the voltage recovery amount VR of thecurve Y is about 0.4 to 0.5 V. A specific manner of determining thevoltage recovery amount VR in FIG. 3 will be described later.

The voltage recovery amount VR obtained here is compared with thelithium precipitation determination threshold. In FIG. 3, the thresholdis set at 0.8 V (3.1 V in cell voltage); however, this is just anexample. In the curve X, the voltage recovery amount VR is higher thanthe threshold, so it may be determined that metal lithium is notprecipitated in the battery group 5. In the curve Y, the voltagerecovery amount VR is lower than or equal to the threshold, so it may bedetermined that metal lithium is precipitated in the battery group 5.Thus, it is possible to determine whether metal lithium is precipitatedin the battery group 5.

Note that the determination that metal lithium is precipitated does notmean that metal lithium is precipitated because of the above describedconstant current discharge; the determination means that metal lithiumhas already been precipitated before the constant current discharge (orpreceding preliminary charge) is started. In addition, as is apparentfrom FIG. 3, determination as to whether metal lithium is precipitatedin the present embodiment may be adequately carried out about 30 minuteslater after the constant current discharge is started.

Next, the reason why it may be determined whether metal lithium isprecipitated through the above described procedure will be describedwith reference to the graph of FIG. 4. FIG. 4 is a graph that shows therelationship between an SOC (state of charge) and potentials of positiveand negative electrodes in the lithium ion secondary battery. In thegraph of FIG. 4, the ordinate axis represents the potentials ofelectrodes, and the abscissa axis represents the SOC of the batteries.The graph of FIG. 4 shows a curve D, a curve E and a curve F. The curveD is a curve for the positive electrode potential. The curve E is acurve for the negative electrode potential in the negative electrode inwhich metal lithium is not precipitated. The curve F is a curve for thenegative electrode potential in the negative electrode in which metallithium is precipitated. Note that the positive electrode potential isnot differentiated depending on whether metal lithium is precipitated.This is because, as described above, metal lithium precipitates in thenegative electrode.

The curve D of the positive electrode potential in FIG. 4 slopes upwardas a whole. Within the curve D, particularly, a slope in a low SOC rangeis steep. The slope corresponds to the magnitude of polarization in thepositive electrode of the lithium ion secondary battery. That is,polarization in the positive electrode of the lithium ion secondarybattery is substantially constant in the range other than the low SOCrange, and is higher in the low SOC range than in the other range.

On the other hand, the curve E of the negative electrode potential (nometal lithium precipitation) slopes downward as a whole. With in thecurve E as well, particularly, a slope in a low SOC range is steep. Theabsolute value of the slope corresponds to the magnitude of polarizationin the negative electrode of the lithium ion secondary battery. That is,polarization in the negative electrode of the lithium ion secondarybattery is substantially constant in the range other than the low SOCrange, and is higher in the low SOC range than in the other range. Inaddition, in the range in which the curve D has large polarization, thecurve E also has large polarization. That is, in the lithium ionsecondary battery in which metal lithium is not precipitated, as thepolarization of the positive electrode increases, the polarization ofthe negative electrode also increases. Furthermore, the curve E in FIG.4 is located below the curve D as a whole. That is, the curve E islocated below the curve D at the left end in FIG. 4 where the SOC islowest.

The curve F in the case where metal lithium is precipitated may bepresumed as the one that is obtained by substantially shifting the curveE slightly rightward in parallel as a whole. Thus, in the range in whichthe curve F has large polarization, the curve D has small polarization.In addition, the curve F substantially overlaps with the curve E in FIG.4 except the range in which the polarization is large.

Voltage measurement described with reference to FIG. 2 and FIG. 3according to the present embodiment is as follows when applied to FIG.4. First, the cell voltage of the lithium ion secondary batterycorresponds to a difference between the positive electrode potential andthe negative electrode potential at the same SOC in FIG. 4. For example,G-G′ is an example of that. Here, it is assumed that G-G′ is a voltage(3.6 V in FIG. 3) at the time when the constant current discharge isstarted (S1 in FIG. 2). Then, when metal lithium is not precipitated,H-H′ corresponds to the voltage VQ (2.3 V at time Z in FIG. 3) at thetime when the constant current discharge is terminated (S3 in FIG. 2);whereas, when metal lithium is precipitated, J-J′ corresponds to thevoltage VQ. The length of H-H′ is equal to the length of J-J′, and thelength of G-G′ is larger than the length of H-H′ or the length of J-J′.In addition, three points G; G′ and J fall within the small polarizationrange in the respective curves; however, three points H, H′ and J′ fallwithin the large polarization range in the respective curves.

Here, focusing on the time at which the constant current discharge isterminated when metal lithium is not precipitated, the points H and H′both fall within the large polarization range as described above. Thatis, both the positive electrode and the negative electrode are greatlypolarized. Large polarization means that the internal resistance of thelithium ion secondary battery is large. In this way, discharge isterminated in a state where the internal resistance is large, so thevoltage recovery amount VR thereafter is large. This is because thevoltage recovery amount VR is substantially proportional to the productof the internal resistance [Ω] of the lithium ion secondary battery atthe time of termination of discharge and the current value [A]immediately before the termination of discharge.

On the other hand, when metal lithium is precipitated, as describedabove, the point J′ falls within the large polarization range; however,the point J falls within the small polarization range. That is, thenegative electrode is greatly polarized, but the positive electrode isnot polarized so much. Therefore, the internal resistance of the lithiumion secondary battery at the time of termination of discharge is smallerthan that when metal lithium is not precipitated. Thus, the voltagerecovery amount VR after termination of discharge is small by that much.This is the reason why it may be determined whether metal lithium isprecipitated on the basis of the voltage recovery amount VR.

Here, the role of the constant current discharge before the voltagerecovery amount VR is acquired is to bring the objective lithium ionsecondary battery into a determinable state. That is, when the SOC ofthe lithium ion secondary battery is high (around the points G and G′ inFIG. 4), there is almost no difference of polarized state depending onwhether metal lithium is precipitated. This is because the curve E andthe curve F in FIG. 4 almost overlap each other in this range.Therefore, it is difficult to make determination in this state.

The SOC of the lithium ion secondary battery is decreased by constantcurrent discharge to around the points H, H′, J and J′ in FIG. 4. By sodoing, there appears a difference of polarized state depending onwhether metal lithium is precipitated. This is because the curve E andthe curve F in FIG. 4 do not overlap each other in this range. Thevoltage recovery amount VR is acquired in this state, so it is possibleto determine whether metal lithium is precipitated. In other words, therole of the constant current discharge is to bring the lithium ionsecondary battery into a low state of charge to an extent such thatthere appears a difference of polarized state depending on whether metallithium is precipitated.

Note that the internal resistance of the lithium ion secondary batteryhas temperature dependency. That is, the internal resistance tends to belarger at low temperatures than at high temperatures. This means thatthe above described voltage recovery amount VR tends to increase at lowtemperatures as compared with at high temperatures. Therefore, thelithium precipitation determination threshold to be compared with thevoltage recovery amount VR desirably has temperature dependency. Thatis, a large threshold is used at low temperatures as compared with athigh temperatures. Conversely, a small threshold is used at hightemperatures as compared with at low temperatures. In order to implementthe above configuration, it is only necessary that a map that defines athreshold applied at each temperature of the battery group 5 is storedin the processing unit 4. Of course, in the map, thresholds specifiedfor low temperatures should be larger than thresholds specified for hightemperatures. Then, an appropriate threshold is selected from among thethresholds in the map on the basis of the temperature acquired by thethermometer 11.

Alternatively, instead of selecting the threshold to be compared withthe voltage recovery amount VR from the map on the basis of thetemperature of the batteries, the acquired voltage recovery amount VRitself may be corrected on the basis of the temperature of the batterygroup 5. That is, the voltage recovery amount VR is corrected to asmaller value as the battery temperature decreases; whereas the voltagerecovery amount VR is corrected to a larger value as the batterytemperature increases. After that, the corrected voltage recovery amountVR is compared with the threshold. Through the above manner as well, itis possible to handle the temperature dependency of the internalresistance of the lithium ion secondary battery.

Next, a method of determining the voltage recovery amount VR in FIG. 3will be described. Some of determining methods are conceivable; however,any determining method may be used as long as the same method is usedeach time. The simplest method is based on a fixed waiting time. Thatis, a waiting time for sampling the voltage between both ends of thebattery group 5 after time Z is determined in advance in order toacquire the voltage recovery amount VR. By subtracting the dischargetermination voltage VQ per cell from the cell voltage after a lapse ofthe waiting time, the voltage recovery amount VR may be determined. InFIG. 3, a period of time during which the voltage steeply increasesafter time Z is slightly shorter than 100 seconds, so it is onlynecessary that the waiting time is set to a period of time longer thanor equal to 100 seconds.

Another determining method focuses on an increase in cell voltage. Thatis, after time Z, the voltage between both ends of the battery group 5is periodically repeatedly sampled. Then, the voltage between both endsof the battery group 5 increases each time it is sampled; however, anincrease in the voltage becomes extremely small as the curve after timeZ in FIG. 3 becomes closer to a horizontal line. Then, a threshold(voltage recovery amount determination threshold) is set for an increasebetween adjacent voltage values sampled at a constant time interval. Thevoltage recovery amount VR may be determined on the basis of the cellvoltage at which the increase in voltage is smaller than or equal to thethreshold.

Another conceivable method is, for example, a determining method inwhich the curve of a change in voltage value after time Z isapproximated to a function that converges to a limit value and then thevoltage recovery amount VR is determined on the basis of the limitvalue. Any of these generally known methods is applicable.

A lithium ion secondary battery equipped for a vehicle is conceivable asa major application of the method of determining whether metal lithiumis precipitated in a lithium ion secondary battery according to thepresent embodiment. The vehicle may be any vehicle that entirely orpartially uses electric energy from a lithium ion secondary battery asthe power source. The vehicle may be, for example, an electricautomobile, a hybrid automobile, a plug-in hybrid automobile, a hybridrailroad vehicle, a forklift, an electric wheelchair, an electricpower-assisted bicycle, an electric scooter, or the like.

That is, determination may be made in such a manner that the lithiumprecipitation determination system 1 shown in FIG. 1 is externallyconnected to the in-vehicle lithium ion secondary battery.Alternatively, determination may be made by the vehicle by itself insuch a manner that a control unit of the vehicle incorporates thefunction of the lithium precipitation determination system 1 shown inFIG. 1.

An example of such a vehicle is shown in FIG. 5. The vehicle 400 is ahybrid automobile that is driven by using an engine 440 and a motor 420in combination. The vehicle 400 includes a vehicle body 490, the engine440, the motor 420 assembled to the engine 440, a cable 450, a controlunit 430 and a battery pack 401 that contains a plurality of batteriesinside. The control unit 430 incorporates not only an inverter, or thelike, for driving the motor 420 but also the function of the lithiumprecipitation determination system 1 shown in FIG. 1. However, the motor420 in the vehicle 400 serves as the charging unit 8 and the dischargingunit 9 among the elements of FIG. 1.

In the case of the vehicle 400, the voltage recovery amount VR in FIG. 3is desirably about 1 [V]. This is to reliably start the engine 440.However, when the battery group 5 degrades because of precipitation ofmetal lithium, it may result in a situation that the voltage recoveryamount VR becomes only about 0.5 [V]. When such a state is detected bythe determining method according to the present embodiment, it ispossible to prompt a user to take appropriate measures, such as batteryreplacement.

Here, in the vehicle 400, usually, it is not assumed to drive the motor420 through constant voltage control. Therefore, the control unit 430mostly does not have a constant voltage control function. However, it isnot inconvenient because of that. This is because, in the methodaccording to the present embodiment, constant current control is usedbut constant voltage control is not used. Of course, for some otherreasons, the control unit 430 may have a constant voltage controlfunction.

In addition, a lithium ion secondary battery equipped for a device,other than a vehicle, that uses a battery as at least one of energysources may be set as a determination object. Such a device may be, forexample, various household electrical appliances, office equipment andindustrial equipment, such as a personal computer, a cellular phone, abattery-powered electric tool and an uninterruptible power supply. Inaddition, an electric cell that is not formed into a battery pack may beset as a determination object.

As described in detail above, according to the present embodiment, thelithium ion secondary battery performs constant current discharge untilthe battery voltage has decreased to the discharge termination voltageVQ and then acquires the voltage recovery amount VR after termination ofthe discharge. The voltage recovery amount VR depends on the internalresistance of the lithium ion secondary battery at the time oftermination of discharge as described above, and the internal resistancevaries on the basis of whether metal lithium is precipitated in thelithium ion battery. Therefore, it may be determined whether metallithium is precipitated in the lithium ion battery on the basis of thevoltage recovery amount VR. Thus, the system and method that are able todetermine whether metal lithium is precipitated in the lithium ionsecondary battery and the vehicle equipped with the system areimplemented. Here, determination according to the temperature of thebattery is possible.

The invention has been described with reference to example embodimentsfor illustrative purposes only. It should be understood that thedescription is not intended to be exhaustive or to limit form of theinvention and that the invention may be adapted for use in other systemsand applications. The scope of the invention embraces variousmodifications and equivalent arrangements that may be conceived by oneskilled in the art.

1-7. (canceled)
 8. A determination method for determining whether metallithium is precipitated in a lithium ion secondary battery, comprising:causing the lithium ion secondary battery to perform constant currentdischarge; terminating the constant current discharge when the lithiumion secondary battery has a predetermined low state of charge throughthe constant current discharge; acquiring a natural increase in voltageof the lithium ion secondary battery after the constant currentdischarge is terminated; comparing the acquired natural increase with apredetermined threshold; and determining that the metal lithium is notprecipitated when the natural increase is larger than or equal to thethreshold, and determining that the metal lithium is precipitated whenthe natural increase is smaller than the threshold.
 9. The determinationmethod according to claim 8, further comprising increasing the thresholdfor comparison with the natural increase as a temperature of the lithiumion secondary battery decreases, and decreasing the threshold forcomparison with the natural increase as the temperature of the lithiumion secondary battery increases.
 10. The determination method accordingto claim 8, further comprising: correcting the acquired natural increaseto a smaller value as a temperature of the lithium ion secondary batterydecreases, and correcting the acquired natural increase to a largervalue as the temperature of the lithium ion secondary battery increases;and comparing the corrected natural increase with the threshold.
 11. Thedetermination method according to claim 8, further comprising: ahead ofthe constant current discharge, determining whether a voltage of thelithium ion secondary battery is higher than or equal to a predetermineddetermination start allowable voltage; and when it is determined thatthe voltage of the lithium ion secondary battery is higher than or equalto the determination start allowable voltage, the constant currentdischarge is immediately started; whereas, when it is determined thatthe voltage of the lithium ion secondary battery is lower than thedetermination start allowable voltage, the lithium ion secondary batteryis charged to a predetermined target charge voltage and then theconstant current discharge is started.
 12. The determination methodaccording to claim 11, wherein the determination start allowable voltagecorresponds to a 60 to 70 percent state of charge of the lithium ionsecondary battery.
 13. The determination method according to claim 8,wherein the predetermined low state of charge is a 10 to 20 percentstate of charge of the lithium ion secondary battery.